netdev - IP header identification field is zero, why?

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Date:	Wed, 7 Oct 2009 22:04:34 +0800
From:	thomas yang <lampsu@...il.com>
To:	netdev@...r.kernel.org
Subject: IP header identification field is zero, why?

This is captured on my PC (Fedora 11, Linux）

[root@...alhost ~]# tcpdump -i eth1 icmp -n -x
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 96 bytes
17:02:39.025882 IP 192.168.1.64 > 192.168.1.1: ICMP echo request, id
25096, seq 1, length 64
	0x0000:  4500 0054 0000 4000 4001 b717 c0a8 0140
......
17:02:39.027866 IP 192.168.1.64 > 192.168.1.1: ICMP echo request, id
25096, seq 2, length 64
	0x0000:  4500 0054 0000 4000

The  IP header 'identification' field is zero, why?

I wrote a simple UDP server and UDP client programs, and captured some
packets, the IP identification is also zero.

Should the host increase this field for each packet it sends?

I captured some TCP packets, all of the IP identification are
different, not zero.

--
thomas
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